if-options: fix NULL dereference on missing variable name

[iliasabk]

Summary

parse_option() crashes with a NULL-pointer dereference when a define/define6/definend directive's variable-name field is empty or whitespace-only (fixes #731).

Root cause

After the type token is consumed, the variable name is read with

arg = strskipwhite(fp);
fp = strwhite(arg);
if (fp)
    *fp++ = '\0';
if (strcasecmp(arg, "reserved")) {

strskipwhite() returns NULL when the remainder is empty or whitespace-only, and strcasecmp(NULL, "reserved") then segfaults.

Reproducer

A config file whose line ends with an escaped space (which the trailing-whitespace trim in read_config() deliberately preserves) leaves fp pointing at a NUL byte:

define 119 string=x\

(with a literal space after the backslash, before the newline)

AddressSanitizer: SEGV in strcasecmp
    #1 parse_option if-options.c:2120
    #2 parse_config_line if-options.c:2680
    #3 read_config if-options.c:3023

Verified locally with an ASan build (./configure --enable-debug --without-openssl, -fsanitize=address): unpatched crashes at if-options.c:2120, patched prints type requires a variable name.

Fix

Treat a NULL variable name the same as an absent one (the !fp branch): reject it for types that require a name, and accept np == NULL for OT_OPTION. No behaviour change for well-formed directives — a define 119 string myvar and define 120 option reserved still parse cleanly.

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Walkthrough

parse_option now validates missing variable names in both parsing branches. Only O_ENCAP definitions may retain a null variable name. Other nameless definitions log an error and return -1.

Changes

Option Definition Validation

Layer / File(s)	Summary
Validate option variable names src/if-options.c	Both missing-name branches reject non-O_ENCAP definitions before reserved-name handling. O_ENCAP definitions may retain a null variable name.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Medium

Merge Risk: ⚪ Minimal · up to 1cbcc

Missing or whitespace-only option names are rejected safely outside encapsulated option references, while valid encapsulated references remain supported. No actionable merge risk remains.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the NULL dereference fix in if-options and the missing variable name condition.
Description check	✅ Passed	The description directly explains the crash, root cause, reproducer, fix, and preserved behavior for valid directives.
Linked Issues check	✅ Passed	The change addresses issue #731. parse_option() rejects a missing variable name before strcasecmp() can receive NULL. The change covers both empty and whitespace-only input paths. It permits a n…
Out of Scope Changes check	✅ Passed	The reported change is limited to the parse_option() handling of missing variable names. The additional restriction for nameless OT_OPTION entries directly prevents the later null-variable failure…

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Actionable comments posted: 1

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Inline comments:
In `@src/if-options.c`:
- Around line 2120-2126: Update the arg == NULL handling in the option parser to
permit nameless OT_OPTION entries only when the current option type is O_ENCAP;
reject O_DEFINE and O_EMBED entries through the existing error path before
storing opt->var as NULL, while preserving named-option behavior.

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